Using a True RMS Voltmeter

Transcription

1 The meter and probe combination can be purchased at wholesale distributors in most cities and can vary in cost from $400 to $500. While that may seem like a steep investment, the cost is relatively small when confronted with a difficult situation that only this equipment can identify. The meter itself is multi-functional, allowing the operator the flexibility to test for both the mundane (supply voltage) and the complex (lamp current). The high voltage probe is essential for measuring voltages above 600 volts, a must when measuring the output voltages on the secondary side of neon

2 transformers. In any case and under all circumstances, using this equipment should be left to those with a working knowledge of electricity, building wiring and a basic understanding of electric sign operation. Basic Use #1: Measure Supply Voltage Supply voltage is the most overlooked problem in the sign installation and maintenance business. Whether the voltage supplied to the sign is supposed to be 120 or 277 volts, many times it is assumed to be the correct voltage, proper polarity and while possessing a proper ground. There is only one way to be sure measure it! First, check the plate voltage of the sign ballast or neon transformer in the sign and then start by measuring the voltage across the white (neutral) supply lead and the black (hot) supply lead. If the display on the meter reads more than plus/minus 10% of the input voltage listed on the ballast or transformer label, you may have operating issues when the sign is energized. Ground, sometimes known as an earth ground, (not bonding), is a critical element of any safe electrical circuit. Recent technologies have made this component of the circuit more critical and in some instances its absence can cause perplexing problems during the sign operation. The most common way to identify the ground is a green wire (sometimes bare copper) that is supplied with the customary black and white supply wires that originate at the electrical panel. Be careful here just because the ground wire is there, do not assume that it is serving the grounding function. Again, measure! First take a voltage measurement between the black (hot) lead and the ground wire. The digital display should read exactly your supply voltage (measured above). The second measurement is taken between the white (neutral) lead and the ground wire. The digital display should read zero (0) to few volts. If the readings are not as described above, an electrician should look at the circuitry for possible problems. Just a quick note on bonding: Bonding and grounding are two very different issues, which require too much detail for this article. Depending on local code requirements, bonding should be taken up with local electrical inspectors. With the procedures described to check ground, you have also checked for proper polarity of the wiring circuit. In most single branch, three wire circuits, the white lead should be neutral and the black lead should be hot. With old electromagnetic components, polarity or the proper connection of white (neutral) and black (hot) leads to a circuit were not as critical as it has become today with

3 the widespread use of electronics in all sorts of equipment, not just signs. Basic Use #2: Measuring Neon Transformer Output To re-emphasize safety, electric voltage, especially high voltages, should only be worked on with the proper tools and personal expertise. To ignore these simple rules could endanger your life. With that said, the procedures for measuring transformer operation requires a high voltage probe in conjunction with the voltmeter used previously to measure supply voltage, ground and polarity. By testing for loaded (with neon attached) or unloaded (open circuit) conditions, the transformer can either be verified as inoperable or eliminated as the source of problems in the sign. Open Circuit Measurements The first procedure determines the most important factor in neon sign installation, repair and maintenance does the transformer work as it is supposed to. Only by MEASURING (recurring theme) can the output be verified. Disconnect the power to the sign and remove the high voltage leads (those going to the neon tubing) from the transformer. Reconnect the power supply to the transformer and energize. With the high voltage probe attached to the voltmeter, measure the voltage at each of the output terminals of the transformer. By adding together the voltages measured and multiplying by two, it can be determined whether the transformer is operating according to its plate voltage. Example: Label reads 12,000 Volts, 30 ma on Transformer To Verify: Measure Output #1 = 6.00 (6000 V) Digital Reading Measure Output #2 = 6.00 (6000 V) Digital Reading (Output #1 + Output #2) = Plate Voltage ( ) = 12,000 Another critical bit of information gathered in this method of measuring transformer output is the balance of high voltage output. For instance, if Output #1 is not equal to Output #2 with plus/minus 10-15% under no load conditions, the transformer is not operating properly. Loaded Circuit Measurements Once the transformer operation is verified, the next steps involve re-energizing the neon and taking more measurements! Reattach the neon tubing to the

4 transformer and connect the power source. Energize and have the voltage meter and probe in hand. Measure the voltage at each terminal following the same procedures from the first section of Open Circuit Measurements. With these two measurements from the high voltage bushings on the neon transformer under load, you can identify two critical factors about the sign operation first, is the neon load and the transformer output properly matched and second, is the neon load properly balanced across the secondary high voltage terminals. Using the above example: Example: Label reads 12,000 Volts, 30 ma on Transformer To Verify: Measure Output #1 = 3.00 (3000 V) Digital Reading Measure Output #2 = 3.00 (3000 V) Digital Reading 2 X (Output #1 + Output #2) = Plate Voltage 2 X ( ) = 12,000 You will notice that under load conditions, a transformer will produce half the plate (label) voltage. As above, the Output #1 and Output # 2 should be equal to each other plus/minus 10-15% and the voltage from either Output should never exceed the plate voltage divided by four. Under loading by more than 20% on either output is not acceptable and can be identified at either Output by dividing plate voltage by four (12,000/4=3000) and multiplying by 80% (3000 x. 80=2400). Under loading causes premature transformer failures. Basic Uses #3: Basic Fluorescent Troubleshooting Here is where the Clamp-on Amp Probe comes in very handy. Simply by toggling through the voltage and amperage capabilities of the meter, any individual can effectively measure sign ballasts output to the lamps. The MEASUREMENTS are simple but access to the socket wiring is critical. At or close to the entry point of the wiring to the fluorescent socket, place the wires individually into round section of the amp probe. While the sign is energized, measure first the amperage and then the voltage in that wire and record it by color on a sheet of paper. Each wire should have no more than 2 to 4 volts flowing through it during operations. Each wire should also have approximately.8 to 1.0 amps being drawn during operation. By comparing all values for each wire, you can identify individual sockets where problems are occurring crossed wires, improper lamp

5 seating, no lamp pre-heating just to name a few. This article has just covered the very basics in the usage of a high quality probe and voltmeter (with amp probe). For more details, ask your distributor or any transformer/ballast manufacturer.